1. Technical Field
The present invention relates to a multilayer mirror etc. used in EUV lithography and, more particularly, to a technique for reducing the incidence angle dependence of the reflectivity on the surface of a mirror.
2. Background of the Prior Art
At present, as a method for manufacturing a semiconductor integrated circuit, reduced projection exposure capable of obtaining a high processing speed is widely utilized. In the reduced projection technique, as the semiconductor integrated circuit device becomes more finer, projection lithography using soft X-rays having a wavelength of about 11 to 14 nm instead of ultraviolet ray is being developed (refer to non-patent document 1). Recently, the technique is also called as the EUV (Extreme Ultra-Violet, soft X-ray) lithography. The EUV lithography is expected as a technique having a resolution of 45 nm or less, which has been impossible to realize with the conventional photolithography (a wavelength of about 190 nm or more).
Meanwhile, in a currently mainstream reduced projection optical system using visible or ultraviolet ray, a lens, which is a transmission type optical element, can be used. A reduced projection optical system for which a high resolution is required is composed of a number of lenses. In contrast to this, in the wavelength band of the EUV ray (soft X-ray), there is no transparent material and the refractive index of a material is very close to 1, therefore, a conventional optical element making use of refraction cannot be used. Instead of this, a grazing incidence mirror making use of total reflection, a multilayer mirror capable of obtaining a high reflectivity as a whole by aligning the phase of weak reflected light at the boundary surface to overlap a number of reflected light rays, etc., are used.
In a projection optical system using a lens, it is possible to realize an optical system in which light advances in one direction along the optical axis, however, in a projection optical system configured by a mirror, a flux of light is turned back many times. Due to this, it becomes necessary to prevent the turned back flux of light from interfering with a mirror substrate spatially and the numerical aperture (NA) in the optical system is restricted.
At present, as a projection optical system, ones composed of four to six mirrors are proposed. In order to attain a sufficient resolution, it is preferable for the numerical aperture of a projection optical system to be large, therefore, an optical system composed of six mirrors capable of attaining a large numerical aperture is considered to be promising. As an example of six-mirror optical system, there is a configuration proposed by Takahashi et al. (refer to the patent document 1 and FIG. 21 to be described later).
In order for a reduced projection optical system to exhibit sufficient performance in reduced projection exposure, the configuration of an illumination optical system is also important. In order for a projection optical system to exhibit a sufficient resolution, it is necessary for irradiation intensity to be uniform in a pupil as well as illuminating an exposure region on a mask on which a circuit pattern to be transferred is formed with uniform intensity.
Further, in order to secure throughput, it is also important to illuminate with the light as strong as possible. As an example of such an illumination optical system, which is disclosed in, for example, the patent document 2.
In the multilayer mirror constituting an EUV optical system, materials suited to obtain a high reflectivity differ depending on the wavelength band of incidence light. For example, in the wavelength band near 13.5 nm, if a molybdenum Mo/Si multilayer in which a molybdenum (Mo) layer and a silicon (Si) layer are laminated by turns is used, a reflectivity of 67.5% can be obtained for vertical incidence. Further, in the wavelength band near 11.3 nm, if a Mo/Be multilayer in which a molybdenum (Mo) layer and a beryllium (Be) layer are laminated by turns is used, a reflectivity of 70.2% can be obtained for vertical incidence (refer to the non-patent document 2). The full width at half maximum (FWHM) of the reflectivity peak of the multilayer reported in the non-patent document 2 is about 0.56 nm in the case of the Mo/Si multilayer the periodic length of which has been adjusted so as to have a peak at a wavelength of 13.5 nm for vertical incidence.
Meanwhile, it is known that the reflectivity of a multilayer mirror varies considerably depending on the optical incidence angle and wavelength. FIG. 19 is a graph showing an example of the incidence angle dependence of the reflectivity of a conventional multilayer mirror. In the drawing, the horizontal axis represents the incidence angle (degree (°)) of the light that is made incident into a multilayer mirror and the vertical axis represents the reflectivity (%) for the EUV ray with a wavelength (λ) of 13.5 nm. As seen from the drawing, in the conventional multilayer mirror, a high reflectivity of 70% or more is obtained when the incidence angle is about 0° to 5°, however, when it is 10° or more, the reflectivity falls considerably.
FIG. 20 is a graph showing an example of the spectral reflectivity properties of a conventional multilayer mirror. In the drawing, the horizontal axis represents the wavelength (λ) of the incidence light and the vertical axis represents the reflectivity (%). Note that, the incidence angle is assumed to be 0° (vertical incidence to the reflective surface). As seen from the drawing, in the conventional multilayer mirror, a high reflectivity of 70% or more is obtained in the vicinity of a wavelength of 13.5 nm (in the central part in the drawing), however, in other wavelength bands other than that, the reflectivity falls considerably.
For such a problem, Kuhlmann et al. has proposed a reflective multilayer having an approximately uniform reflectivity across a wide wavelength band by making uneven the periodic structure (film thickness of each layer) of the reflective multilayer (refer to the non-patent document 3). The non-patent document 3 discloses a structure of a multilayer having a wide band for the reflectivity angle distribution or the spectral reflectivity, which has been obtained by adjusting the thickness of each layer of a 50-layer pair multilayer using a commercially available multilayer optimizing program.
For example, in the case of a multilayer the periodic length of which is constant, if the periodic length is optimized such that the reflectivity is maximum in a vertical incidence arrangement, the range in which a high reflectivity can be kept is when the incidence angle is 0° to 5° and when the incidence angle is 10° or more, the reflectivity falls considerably. In contrast to this, the non-patent document 3 discloses a multilayer having an uneven structure in film thickness, the reflectivity of which becomes almost constant at about 45% in the incidence angle range of 0° to 20°. Although the full width at half maximum (FWHM) of the spectral reflectivity peak of a normal Mo/Si multilayer is about 0.56 nm, the non-patent document 3 also discloses a structure the reflectivity of which becomes almost uniform at 30% across the wavelength range of 13 nm to 15 nm for vertical incidence.
The uniformization of the reflectivity in a wide wavelength band and the uniformization in a wide incidence angle range described above are not the properties that can be controlled individually, and in a multilayer capable of obtaining a uniform reflectivity in a wide wavelength band, there is a trend that the change in reflectivity becomes small even in a wide incidence angle range. A multilayer capable of obtaining a uniform reflectivity in such a wide wavelength range can make use of the EUV ray in a wide wavelength region although the reflectivity peak value is lower than that of a normal multilayer, therefore, it can be expected to be capable of obtaining a large amount of light depending on its applications when the band of the incidence light wavelength is wide.
Further, Singh et al. have reported that by making the ┌ value (the proportion of the periodic length of a multilayer to the thickness of a Mo layer) uneven in the depth direction in a Mo/Si multilayer, the reflectivity is increased (refer to the non-patent document 4). The EUV reflectivity of a Mo/Si multilayer reaches its maximum when the ┌value is 0.35 to 0.4, however, the non-patent document 4 discloses that a more increase in reflectivity can be obtained when bringing the ┌value of Mo/Si close to 0.5 at the portion of the substrate side (deep layer side) of the multilayer than when setting it to a constant value of 0.4 for the entire multilayer.
Meanwhile, as a configuration of a reflective multilayer capable of obtaining a high reflectivity for the EUV ray in the vicinity of a wavelength of 13 nm, Ru/Si is known, in addition to Mo/Si (Ru stands for ruthenium). If it is assumed that n is a refractive index and k is an extinction coefficient (the imaginary part of a complex refractive index), the optical constants (n, k) of silicon at a wavelength of 13.5 nm are
n (Si)=0.9993, and
k (Si)=0.0018.
In contrast to this, the optical constants (n, k) of molybdenum and ruthenium are
n (Mo)=0.9211,
k (Mo)=0.0064,
n (Ru)=0.8872, and
k (Ru)=0.0175,
respectively.
Like a multilayer for the EUV ray, when absorption occurs in the multilayer itself, in order to obtain a high reflectivity, it is preferable that the difference in refractive index of a substance constituting the multilayer be large and absorption be small. As seen from the above-mentioned optical constants, from the standpoint of refractive index, a Ru/Si multilayer is suited and from the standpoint of absorption, a Mo/Si is more suited to obtain a high reflectivity. In the case of these two multilayers, the influence of absorption is dominant and the Mo/Si multilayer has a higher peak reflectivity.
The full width at half maximum of the reflectivity peak of a multilayer is brought about by the difference in refractive index. It is known that the band full width (2Δg) of the peak of the reflectivity of a dielectric multilayer (a multilayer in which two substances having different refractive indices are laminated by turns) well known in infrared, visible, and ultraviolet regions is expressed by the following formula (for example, refer to non-patent document 5).
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                      2          ⁢          Δ          ⁢                                          ⁢          g                =                              4            π                    ·                                    sin                              -                1                                      ⁡                          (                                                                    n                    H                                    -                                      n                    L                                                                                        n                    H                                    +                                      n                    L                                                              )                                                          (        1        )            
Here, nH is the refractive index of a high refractive-index substance and nL is the refractive index of a low refractive-index substance.
As seen from the above formula, the larger the refractive index difference between the two substances constituting a multilayer, the more the band increases, therefore, a wider full width at half maximum can be obtained from a Ru/Si multilayer than that from a Mo/Si multilayer. In the case where there is no absorption by a film, the peak value of the dielectric multilayer reflectivity gradually reaches 100%, however, in the EUV region it does not reach 100% because of absorption.
Since the magnitude of absorption depends on the wavelength, if the change in reflectivity versus wavelength is plotted, the reflectivity is asymmetry before and after the peak wavelength. The peak reflectivity of a multilayer in the EUV region increases as the number of pairs of formed films increases, however, it saturates at a certain number of pairs. The number of pairs with which saturation is reached is about 50 pair layers for a Mo/Si multilayer and about 30 pair layers for a Ru/Si multilayer. The reason that the reflectivity reaches saturation is that by reflection and absorption at each boundary surface when the EUV ray passes through a film, almost no light reaches a position deeper than that and there is no longer contribution to the reflection of the entire film. A Ru/Si multilayer is greater than a Mo/Si multilayer in the magnitude of absorption and the reflectivity at a single boundary surface is higher, therefore, the number of pairs with which saturation is reached is smaller.